The invention relates to an aqueous coating composition based on a mixture of a dispersion of an addition polymer and a rheology modifier. Preferably, this aqueous coating composition is mixed with a metallic pigment, such as aluminum, or a pigment, such as a metal oxide-coated mica, so that coatings with a metallic appearance may be obtained. In this way there is obtained a differential light reflection effect referred to as xe2x80x9cflopxe2x80x9d. A problem with coating systems having a metallic appearance is to obtain a high flop as well as a high gloss.
To obtain a high flop, the metallic pigment on application of the coating composition should be and remain well oriented. To obtain a high gloss, the metallic pigment-containing coating is provided with an unpigmented, so-called clear coat. This system is generally called a xe2x80x9cbase coat/clear coatxe2x80x9d system. In actual practice, the base coat will be sprayed with the clear coat, without prior curing of the base coat (xe2x80x9cwet-on-wetxe2x80x9d). Since the clear coat usually contains organic solvents, steps should be taken to prevent disorientation of the metallic pigment in the base coat as a result of the base coat being weakened up by the organic solvents in the clear coat (xe2x80x9cstrike-inxe2x80x9d).
An aqueous base coat composition is known from EP-A-0 038 127, i.e., a crosslinked core-shell dispersion whereby the shell, when swollen, provides the desired rheological properties. The crosslinking reduces the strike-in. A disadvantage to this system, however, is that the coating composition will have poor film-forming properties, which may manifest itself in poor mechanical properties.
Another aqueous base coat composition is known from EP-A-0 287 144, i.e., a swellable non-crosslinked core-shell dispersion having an amount of (meth)acrylic acid in the shell of 10-60 mole %. Exemplified are swellable non-crosslinked core-shell dispersions having more than 2 wt. % (meth)acrylic acid in 100 parts of the addition polymer. Also in this embodiment a decrease in strike-in is observed.
Both systems disclosed in EP-A-0 038 127 and EP-A-0 287 144 contain in the shell a lot of carboxylic groups, neutralized by an amine to provide the desired rheological properties. However, because of this large amount of salt groups, coatings based on these compositions especially when applied and cured at ambient temperature, show a poor water-resistance.
The present invention now provides an aqueous coating composition which may be used as base coat in a base coat/clear coat system, having good mechanical properties, a high flop, a high gloss, practically no strike-in, and a good water-resistance. Due to the fact that higher solid contents can be achieved with the aqueous coating composition of the present invention, a reduction in drying times and number of coats is obtained. In one or more of these properties the aqueous coating composition of the present invention shows improvement over those disclosed in EP-A-0 038 127 and EP-A-0 287 144.
The aqueous coating composition according to the invention comprises a mixture of
90 to 99 wt. % of a filmforming binder composition comprising an alkali non-swellable core-shell addition polymer dispersion (I), and
1-10 wt. % of a rheology modifying addition polymer dispersion (II),
the sum of the wt. % indicated for the filmforming binder composition and dispersion (II) always being 100 wt. %,
wherein
the polymer dispersion (I) is prepared in two or more steps by emulsion polymerization, and obtained by copolymerization in a first step of
(1) 60-95 parts by weight (calculated on 100 parts by weight of the total addition polymer (I)) of a monomer mixture A consisting of
(i) 65-100 mole % of a mixture of
(a) 10-98 mole % of a (cyclo)alkyl (meth)acrylate of which the (cyclo)alkyl group contains 4-12 carbon atoms,
(b) 0-55 mole % styrene,
(c) 2-15 mole % hydroxy alkyl (meth)acrylate, and
(d) 0-20 mole % of a di(cyclo)alkyl maleate and/or fumarate of which the (cyclo)alkyl groups contain 4-12 carbon atoms,
the sum of the mole % indicated for the monomers (a), (b), (c), and (d) always being 100 mole %, and
(ii) 0-35 mole % of a different copolymerizable monoethylenically unsaturated monomer,
the sum of the mole % indicated for the components (i) and (ii) always being 100 mole %,
and by copolymerization in a subsequent step of
(2) 5-40 parts by weight (calculated on 100 parts by weight of the total addition polymer (I)) of a monomer mixture B consisting of
(e) 1-10 mole % (meth)acrylic acid,
(f) 2-20 mole % hydroxy alkyl (meth)acrylate,
(g) 0-55 mole % styrene, and
(h) 15-97 mole % of a different copolymerizable monoethylenically unsaturated monomer,
the sum of the mole % indicated for the monomers (e), (f), (g), and (h) always being 100 mole %,
with the carboxylic acid groups derived from the (meth)acrylic acid being at least partially neutralized,
resulting in a non-crosslinked addition polymer I,
whereby the total amount of (meth)acrylic acid in 100 parts of the total addition polymer (I) is less than 1.75 wt. %, and
wherein
the polymer dispersion (II) is prepared by emulsion polymerization, and obtained by copolymerization of
(iii) 99.5-99.99 parts by weight (calculated on 100 parts by weight of the total addition polymer (II)) of a monomer mixture C consisting of
(j) 10-80 wt. % (cyclo)alkyl (meth)acrylate,
(k) 20-50 wt. % (meth)acrylic acid,
(m) 0-20 wt. % hydroxyalkyl (meth)acrylate, and
(n) 0-20 wt. % of a different copolymerizable monoethylenically unsaturated monomer,
the sum of the wt. % indicated for the monomers (j), (k), (m), and (n) always being 100 wt. %, and
(iv) 0.01-0.5 parts by weight (calculated on 100 parts by weight of the total addition polymer (II)) of a compound having at least two unsaturated groups,
with the carboxylic acid groups derived from the (meth)acrylic acid being at least partially neutralized.
Preferably, in the first step of the preparation of the polymer dispersion (I), a monomer mixture A is used, consisting of
(i) 80-100 mole %, more preferred 100 mole %, of a mixture of
(a) 30-95 mole % of a (cyclo)alkyl (meth)acrylate of which the (cyclo)alkyl group contains 4-12 carbon atoms,
(b) 0-50 mole % styrene,
(c) 5-12 mole % hydroxy alkyl (meth)acrylate, and
(d) 0-8 mole % of a di(cyclo)alkyl maleate and/or fumarate of which the (cyclo)alkyl groups contain 4-12 carbon atoms, and
(ii) 0-20 mole %, more preferred 0 mole %, of a different copolymerizable monoethylenically unsaturated monomer.
As examples of (cyclo)alkyl (meth)acrylates suitable for use in monomer mixture A and having a (cyclo)alkyl group with 4-12 carbon atoms may be mentioned: butyl acrylate, butyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, octyl acrylate, octyl methacrylate, isobornyl acrylate, isobornyl methacrylate, dodecyl acrylate, dodecyl methacrylate, cyclohexyl acrylate, cyclohexyl methacrylate, and mixtures thereof. Butyl acrylate, butyl methacrylate, and mixtures thereof are preferred.
Examples of hydroxyalkyl (meth)acrylates are 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 4-hydroxybutyl acrylate, 6-hydroxyhexyl acrylate, p-hydroxycyclohexyl acrylate, p-hydroxycyclohexyl methacrylate, hydroxypolyethylene glycol (meth)acrylates, hydroxypolypropylene glycol (meth)acrylates, and mixtures thereof. 2-Hydroxyethyl methacrylate is preferred.
As examples of di(cyclo)alkyl maleates and/or fumarates with the (cyclo)alkyl groups having 4-12 carbon atoms suitable for use in monomer mixture A may be mentioned dibutyl maleate, dibutyl fumarate, 2-ethylhexyl maleate, 2-ethylhexyl fumarate, octyl maleate, isobornyl maleate, dodecyl maleate, cyclohexyl maleate, and mixtures thereof.
As suitable copolymerizable monoethylenically unsaturated monomers to be used in monomer mixture A may be mentioned: alkyl (meth)acrylates having fewer than 4 carbon atoms in the alkyl group, such as methyl methacrylate, methyl acrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, and isopropyl acrylate; alkyl maleates and fumarates having fewer than 4 carbon atoms in the alkyl groups, such as dimethyl maleate, diethyl maleate, diethyl fumarate, and dipropyl maleate; (meth)acrylates having ether groups such as 2-methoxyethyl methacrylate, 2-ethoxyethyl methacrylate, and 3-methoxypropyl acrylate; monovinyl aromatic hydrocarbons, such as vinyl toluene, xcex1-methyl styrene, and vinyl naphthalene; acrylamide and methacrylamide; nitriles such as acrylonitrile and methacrylonitrile; N-alkyl (meth)acrylamide such as N-isopropyl acrylamide, N-isopropyl methacrylamide, N-t-butyl acrylamide, N-t-octyl acrylamide, N,N-dimethyl aminoethyl methacrylate, N,N-diethyl aminoethyl methacrylate; monomers such as vinyl chloride, vinyl acetate, vinyl propionate, and vinyl pyrrolidone, and monomers containing one or more urea or urethane groups, such as for instance the reaction product of 1 mole of isocyanato-ethyl methacrylate and 1 mole of butylamine, 1 mole of benzylamine, 1 mole of butanol, 1 mole of 2-ethylhexanol, and 1 mole of methanol, respectively. Mixtures of these compounds may also be used.
Preferably, in a second step of the preparation of the polymer dispersion (I), a monomer mixture B is used, consisting of
(e) 5-8 mole % (meth)acrylic acid,
(f) 5-12 mole % hydroxy alkyl (meth)acrylate,
(g) 0-30 mole % styrene, and
(h) 50-90 mole % of a different copolymerizable monoethylenically unsaturated monomer.
Examples of hydroxy alkyl (meth)acrylates have been mentioned above. 2-hydroxyethyl methacrylate is preferred.
Examples of copolymerizable monoethylenically unsaturated monomers which may be used in the monomer mixture B include the examples mentioned above for copolymerizable monoethylenically unsaturated monomers which may be used in the monomer mixture A. Also included are (cyclo)alkyl (meth)acrylates having a (cyclo)alkyl group with 4-12 carbon atoms. Examples thereof are also mentioned above. Mixtures of these compounds may also be used. Preferably, the copolymerizable monoethylenically unsaturated monomers are selected from methyl methacrylate, butyl acrylate, butyl methacrylate, and mixtures thereof.
Preferably, polymer dispersion (I) is prepared by emulsion polymerization of
(1) 70-90, preferably 75-85, parts by weight of monomer mixture A and
(2) 10-30, preferably 15-25, parts by weight of monomer mixture B.
Optionally, different monomer mixtures A and/or B may be used successively.
Since the addition polymer (I) is non-crosslinked, the choice of the monomers in monomer mixtures A and B is such that the functional groups present other than the unsaturated bonds cannot react with each other at the reaction conditions for the preparation of the addition polymer.
It is required that the total amount of (meth)acrylic acid in 100 parts of the total addition polymer (I) is less than 1.75 wt. %, preferably less than 1.5 wt. %, more preferably between 0.5-1.4 wt. %. In this manner, the polymer dispersion (I) is non-swellable. The acid value is 3 to 10 mg KOH/g, preferably 5 to 8 mg KOH/g.
The addition polymer (I) has a Mn of from 50 000 to 2 000 000, preferably from 100 000 to 1 000 000.
Preferably, monomer mixture C used in the preparation of the polymer dispersion (II) consists of
(j) 50-70 wt. % (cyclo)alkyl (meth)acrylate,
(k) 30-40 wt. % (meth)acrylic acid,
(m) 0-5 wt. % hydroxyalkyl (meth)acrylate, and
(n) 0-5 wt. % of a different copolymerizable monoethylenically unsaturated monomer.
Preferably, the polymer dispersion (II) is prepared by emulsion polymerization of
(iii) 99.85-99.95 parts by weight of monomer mixture C and
(iv) 0.05-0.15 parts by weight of a compound having at least two unsaturated groups.
Preferably, the (cyclo)alkyl (meth)acrylates in monomer mixture C have alkyl groups with 1-4 carbon atoms. Examples include methyl methacrylate, methyl acrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, sec-butyl acrylate, sec-butyl methacrylate, t-butyl acrylate, t-butyl methacrylate, and mixtures thereof. Preferred are methyl acrylate, ethyl acrylate, and propyl acrylate.
Examples of hydroxyalkyl (meth)acrylates and copolymerizable monoethylenically unsaturated monomer to be used in monomer mixture C are given above for monomer mixtures A and B.
Examples of the compound having at least two unsaturated groups include divinyl toluene, divinyl benzene, trivinyl benzene, divinyl naphthalene, ethylene glycol di(meth)acrylate, trimethylene glycol di(meth)acrylate, 2-ethyl hexane-1,3-dimethacrylate, divinyl xylene, divinyl ethyl benzene, divinyl ether, divinyl sulfone, allyl ethers of polyhydric compounds, such as glycerol, pentaerythritol, sorbitol, sucrose, and resorcinol, allyl ethers of polyisocyanate compounds, such as triallyl isocyanurate, divinyl ketone, divinyl sulfide, allyl (meth)acrylate, diallyl maleate, diallyl fumarate, diallyl phthalate, diallyl succinate, diallyl carbonate, diallyl malonate, diallyl oxalate, diallyl adipate, diallyl sebacate, diallyl tartrate, diallyl silicate, triallyl citrate, triallyl phosphate, and N,Nxe2x80x2-methylene di(meth)acrylamide. Preferred is a compound having at least two unsaturated groups of which at least one is an allylic group. More preferred are diallyl phthalate, allyl methacrylate, and triallyl isocyanurate.
The addition polymer (II) has an acid value of 175 to 350 mg KOH/g, preferably 200 to 300 mg KOH/g, and a hydroxyl value of 0 to 150 mg KOH/g, preferably 0 to 100 mg KOH/g.
By emulsion polymerization is meant here the polymerization of the monomer mixtures of ethylenically unsaturated monomers in water in the presence of a water-soluble or -insoluble initiator and 0.1-5 wt. %, preferably 0.3-2.5 wt. % (calculated on the total monomer mixture(s)) of an emulsifier. The polymer dispersion (I) may be prepared by emulsion polymerization as disclosed in EP-A-0 287 144. The polymer dispersion (II) may be prepared by emulsion polymerization as disclosed in GB 870 994.
The emulsifiers of which use is preferably made in the emulsion polymerization are of an anionic and/or non-ionic nature. Examples of anionic emulsifiers include: potassium laurate, potassium stearate, potassium oleate, sodium decyl sulphate, sodium dodecyl sulphate, sodium dodecylbenzene sulphonic acid, and sodium rosinate. Examples of non-ionic emulsifiers include: linear and branched alkyl and alkylaryl polyethylene glycol and polypropylene glycol ethers and thioethers, alkyl phenoxypoly(ethyleneoxy) ethanols such as the adduct of 1 mole of nonyl phenol and 3-12 moles of ethylene oxide; alkyl (ethyleneoxy) ethanols with 8-18 carbon atoms in the alkyl groups, such as the adduct of 1 mole dodecanol and 3-12 moles of ethylene oxide. Examples of emulsifiers comprising anionic and non-ionic groups are the ammonium or sodium salt of the sulphate of alkyl phenoxypoly(ethyleneoxy) ethanols, such as the adduct of 1 mole of nonyl phenol and 3-12 moles of ethylene oxide, and the ammonium or sodium salt of the sulphate of alkyl (ethyleneoxy) ethanols with 8-18 carbon atoms in the alkyl groups, such as the adduct of 1 mole C12-14 alcohol and 3-12 moles of ethylene oxide. Preferred is the ammonium or sodium sulphate salt of the adduct of 1 mole C12-14 alcohol and 3-12 moles of ethylene oxide.
Also, in emulsion polymerization, the conventional radical initiators may be used in the usual amounts. Examples of suitable radical initiators include water-soluble initiators, such as ammonium persulphate, sodium persulphate, potassium persulphate, and t-butyl hydroperoxide, and water-insoluble initiators, such as bis(2-ethylhexyl) peroxydicarbonate, di-n-butyl peroxydicarbonate, t-butyl perpivalate, cumene hydroperoxide, dibenzoyl peroxide, dilauroyl peroxide, 2,2xe2x80x2-azobisisobutyronitrile, and 2,2xe2x80x2-azobis-2-methylbutyronitrile. As suitable reducing agents which may be used in combination with e.g. a hydroperoxide may be mentioned: ascorbic acid, sodium sulphoxylate formaldehyde, thiosulphates, bisulphates hydrosulphates, water-soluble amines such as diethylene triamine, triethylene tetramine, tetraethylene pentamine, N,Nxe2x80x2-dimethyl ethanol amine, and N,N-diethyl ethanol amine, and reducing salts such as cobalt, iron, nickel, and copper sulphate. Optionally, a chain length regulator, for instance n-octyl mercaptan, dodecyl mercaptan, and 3-mercaptopropionic acid, may also be used.
Copolymerization of the monomer mixtures generally is carried out at atmospheric pressure at a temperature of 40-100xc2x0 C., preferably 60-90xc2x0 C., in an atmosphere of an inert gas, such as nitrogen. Optionally, however, copolymerization may also be carried out at elevated pressure. The reaction conditions for monomer mixtures A and B should however be chosen in such manner that functional groups present in the monomer mixtures other than the unsaturated bonds cannot react with each other.
According to the invention the carboxylic acid groups derived from the acrylic acid and/or methacrylic acid are at least 40-100% neutralized by the addition of a neutralizing agent. As suitable neutralizing agents for the carboxylic acid may be mentioned ammonia and amines such as N,N-dimethyl ethanol amine, N,N-diethyl ethanol amine, 2-(dimethyl)-amino-2-methyl-1-propanol, triethyl amine, and morpholine. It is preferred that the neutralizing of the carboxylic acid groups should be carried out after the polymerization.
The coating composition according to the invention comprises preferably a mixture of 92-95 wt. % of a filmforming binder composition comprising an alkali non-swellable core-shell addition polymer dispersion (I), and 5-7.5 wt. % of a rheology modifying addition polymer dispersion (II).
The coating composition of the present invention consists essentially of water, being an aqueous coating composition. However, about 20 wt. % of liquid content of the coating composition may be an organic solvent. As suitable organic solvents may be mentioned such ether group-containing alcohols as hexylglycol, butoxyethanol, 1-methoxy-propanol-2, 1-ethoxy-propanol-2, 1-propoxy-propanol-2, 1-butoxy-propanol-2, and 1-isobutoxy-propanol-2; alcohols, such as methanol, ethanol, propanol, butanol, pentanol, and hexanol; diols, such as ethylene glycol and diethylene glycol.
The coating composition according to the present invention may be cured by physical drying. Alternatively, however, the coating compositions may be cured in the presence of a curing agent which reacts with hydroxyl and/or carboxyl groups.
Examples of suitable curing agents include N-methylol and/or N-methylol ether groups-containing aminoplastes obtained by reacting an aldehyde, for instance formaldehyde, with an amino or amido groups-containing compound such as melamine, such as Cymel 328, ex Cytec, urea, N,Nxe2x80x2-ethylene urea, dicyanodiamide, and benzoguanamine. The resulting compounds are preferably wholly or partially etherified with alcohols having 1-6 carbon atoms, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, amyl alcohol, hexanol, or mixtures thereof. Especially favourable results may be obtained when using a methylol melamine having 4-6 methyl groups per molecule of melamine, at least 3 methylol groups being etherified with butanol or a butanol etherified condensation product of formaldehyde and N,Nxe2x80x2-ethylene diurea. Examples of other suitable curing agents include polyisocyanates or water-dispersible blocked polyisocyanate such as a methyl ethyl ketoxime-blocked, isocyanate group-containing adduct of a polyisocyanate to a hydroxycarboxylic acid, e.g. dimethylol propionic acid, and aliphatic or aromatic carbodiimides.
In addition to the alkali non-swellable core-shell addition polymer dispersion (I), the filmforming binder composition may also comprise water-dilutable materials such as alkyd resins, polyesters, polyurethanes, and mixtures thereof.
Preferably, the water dilutable material is a polyurethane. The filmforming binder composition may comprise 0.1 to 100 wt. % of the alkali non-swellable core-shell addition polymer dispersion (I) and 99.9 to 0 wt. % of at least one water dilutable material, wherein the sum of the wt. % indicated for dispersion (I) and the water dilutable material(s) is always 100 wt. %. More preferably, the filmforming binder composition may comprise 1 to 99 wt. % of the alkali non-swellable core-shell addition polymer dispersion (I) and 99 to 1 wt. % of at least one water dilutable material. Most preferably, the filmforming binder composition may comprise 25 to 75 wt. % of the alkali non-swellable core-shell addition polymer dispersion (I) and 75 to 25 wt. % of at least one water dilutable material.
In addition, the coating composition may contain the conventional additives and adjuvants, such as pigments, dispersing agents, dyes, and accelerators for the curing reaction. The applicable pigments may have an acid, a neutral or a basic character. Optionally, the pigments may be pre-treated to modify the properties. Examples of suitable pigments include metallic pigments such as aluminum and stainless steel; nacreous pigments, such as mica coated with a metal oxide such as iron oxide and/or titanium dioxide; inorganic pigments, such as titanium dioxide, iron oxide, carbon black, silica, kaolin, talc, barium sulphate, lead silicate, strontium chromate, and chromium oxide; and organic pigments, such as phthalocyanine pigments.
The solids content of the coating composition ranges from 5-60 wt. %, preferably from 10-40 wt. %. This depends on whether a metallic pigment is used or not. The presence of metallic pigments results in a lower solid content compared to the presence of non-metallic pigments. However, compared to conventional aqueous base coat systems, the solid content of the coating composition of the present invention is in both cases higher.
Preferably, the coating composition according to the present invention is used as a base coat in a so-called base coat/clear coat system to provide a high gloss metallic appearance. To this end the coating composition according to the invention comprises so-called xe2x80x9cnon-leafingxe2x80x9d aluminum paste or some other metallic pigment. Use of the coating compositions according to the invention as a base coat may prevent the base coat from being softened by the clear coat after being sprayed with it, so that the metallic effect will not be lost.
The clear coat used in the base coat/clear coat system may for instance be a clear baking lacquer of a conventional polyacrylate/melamine composition. The clear coat may also be a two-component polyester or polyacrylate/polyisocyanate composition. The polyisocyanate may be for example the trimer of 1,6-hexamethylene diisocyanate.
The coating composition according to the invention may be applied to a substrate in any desirable manner, such as by roller coating, spraying, brushing, sprinkling, flow coating, dipping, electrostatic spraying, or electrophoresis, preferably by spraying.
Suitable substrates may be made of wood, metal, and synthetic material. Curing may be carried out at ambient temperature or, optionally, at elevated temperature to reduce the curing time. Optionally, the coating composition may be baked at higher temperatures in the range of, for instance, 60 to 160xc2x0 C., in a baking oven over a period of 10 to 60 minutes. The clear coat may be applied wet-on-wet on the base coat. Optionally, the base coat may be partially cured prior to the application of the clear coat. Also, the base coat may be fully cured prior to the application of the clear coat.
The compositions are particularly suitable in the preparation of coated metal substrates, such as in the refinish industry, in particular the body shop, to repair automobiles and transportation vehicles and in finishing large transportation vehicles such as trains, trucks, buses, and aeroplanes. The compositions of the present invention may also be used in the first finishing of automobiles.
The invention will be further described in the following examples, which must not be construed as limiting the scope of the present invention.